It is known in the art that the copolymerization of cyclic ethers, notably dioxolane and ethylene oxide with formaldehyde, forms polyoxymethylene copolymers with improved thermal and base stability compared to those of polyoxymethylene homopolymer. For most applications, the desired composition range comprises ca. 0.5 to 2.5 mole % of comonomers; enough to provide stability but not so much as to degrade physical and mechanical properties beyond the point of function in most acetal applications. In performing the research which led to the present invention, the inventors discovered that the cyclic ether concentration in the hydrocarbon reaction medium had to be in the range of 0.5-1.0 M in order to achieve comonomer concentration in the copolymer of 0.5-2.5 mol-%. Dioxolane concentrations of that magnitude in the solvent represent a disposal or recycle problem. In addition, with dioxolane there is formation of an unwanted side product--trioxepane to equilibrium levels of 0.3-2% by weight (0.02-0.15 M). The equilibrium concentration of the trioxepane is directly proportional to the dioxolane concentration in the solvent. The present invention avoids the recycle problem by improving the rate of cyclic ether, e.g., dioxolane, incorporation into the polymer by the addition of organic nitro compounds into the reaction. The process thus enhances the selectivity for the cyclic comonomer.
Most common polyoxymethylene copolymerizations use 1,3,5-trioxane and dioxolane as comonomers and Lewis acid initiators. Copolymerization of formaldehyde with cyclic ethers is less common. Polyoxymethylene can be produced from anhydrous formaldehyde anionically, via a reversible polymerization effected in a hydrocarbon slurry. The low solubility of formaldehyde in hydrocarbons and the formation of highly crystalline, insoluble polymer particles drive the equilibrium to both high conversion and high molecular weight. Under cationic catalysis, cyclic ether comonomers can copolymerize with formaldehyde in the hydrocarbon slurry process.
French Patent 1,500,156, assigned to British Industrial Plastics Ltd., issued in 1967, discloses the copolymerization of trioxane and dioxolane initiated by a solution of arsenic pentafluoride initiators in nitro-substituted hydrocarbon solvents. The total concentration of nitro compounds in the reaction mixture is well under 1%.
Usa et al, Japanese Patent Application SHO 48-877, published in 1973, disclose copolymerization of trioxane and cyclic ethers catalyzed by P.sub.2 O.sub.5, the entire reaction taking place in nitro-substituted hydrocarbon solutions such as nitrobenzene and nitromethane.
C. Chen, Canadian Patent 864041, issued 1971, discloses copolymerization of trioxane and dioxolane to molecular weights up to 500M by addition of CH.sub.2 Cl.sub.2 "enhancing agent". It is explicitly stated that nitrobenzene prevents reaction. Preferred initiators include triphenyl, and trimethoxyphenyl carbonium, and trialkyl oxonium salts of PF.sub.6 and AsF.sub.6.
K. Burg et al, Makro. Chem. 142, 247 (1971) disclose the copolymerization of trioxane and dioxolane with, among others, BF.sub.3 etherate initiator. They state explicitly that the incorporated fraction of dioxolane corresponds to the monomer ratio of trioxane to dioxolane.
S. A. Vol'fson et al, Dokl. Akad. Nauk. SSR, 234, 1365 (1977) disclose the copolymerization of HCHO and dioxolane at elevated pressure in ethyl benzene solution, initiated by, among others, BF.sub.3 etherate. It is stated explicitly that comonomer incorporation depends upon the concentration of initiator.
R. S. Jones, Polym. Preprints, 21, 144 (1980) discloses copolymerization of HCHO and dioxolane in hydrocarbon slurry initiated by BF.sub.3 etherate. It is stated explicitly that comonomer incorporation is dependent upon initiator concentration only and shows that dioxolane incorporation into the copolymer is favored in the least polar solvent, hexane. Jones cites a preferred dioxolane concentration of 0.5 M (5.4 wt %) to achieve 1-2 mole % in the copolymer.
Yamashita et al., (J. Polymer Sci., A2, Vol. 4, p 2121-2135, 1966) states that relative reactivity correlates best with basicity and free energy. Copolymerizations involving dioxolane show only small changes in reactivity ratios with change in initiator or reaction medium which ranged from toluene to nitrobenzene. Even in carbocationic copolymerization of trioxane, where much more data exists and reactivity ratios are significantly affected by solvent and catalyst, they state, "a systematic interpretation is difficult even in the simplest cases".
The copolymerization of formaldehyde and dioxolane is further complicated by the heterogeneous nature of the polymerization where the reactive sites reside on or near the surface of the crystalline, insoluble particles.
The art, in general, teaches away from employing nitro-substituted hydrocarbons. Burg, Vol'fson, and Jones all teach that comonomer incorporation depends only upon catalyst concentration or monomer ratio. While not mutually consistent, all seem to exclude the benefit of any additive to enhance the comonomer incorporation. Chen does teach the benefit of an enhancing agent, but the agent is CH.sub.2 Cl.sub.2, the initiators do not include BF.sub.3 etherate, the reaction involves trioxane not HCHO, and, most importantly, Chen explicitly states that nitrobenzene prevents reaction.